Vibrating sensors, such as for example, vibrating densitometers and Coriolis flow meters are generally known and are used to measure mass flow and other information for materials flowing through a conduit in the flow meter. Exemplary Coriolis flow meters are disclosed in U.S. Pat. No. 4,109,524, U.S. Pat. No. 4,491,025, and Re. 31,450 all to J. E. Smith et al. These flow meters have one or more conduits of straight or curved configuration. Each conduit configuration in a Coriolis mass flow meter has a set of natural vibration modes, which may be of simple bending, torsional, or coupled type. Each conduit can be driven to oscillate at a preferred mode.
Material flows into the flow meter from a connected pipeline on the inlet side of the flow meter, is directed through the conduit(s), and exits the flow meter through the outlet side of the flow meter. The natural vibration modes of the vibrating, material filled system are defined in part by the combined mass of the conduits and the material flowing within the conduits.
When there is no flow through the flow meter, a driving force applied to the conduit(s) causes all points along the conduit(s) to oscillate with identical phase or a small “zero offset”, which is a time delay measured at zero flow. As material begins to flow through the flow meter, Coriolis forces cause each point along the conduit(s) to have a different phase. For example, the phase at the inlet end of the flow meter lags the phase at the centralized driver position, while the phase at the outlet leads the phase at the centralized driver position. Pick-off sensors on the conduit(s) produce sinusoidal signals representative of the motion of the conduit(s). Signals output from the pick-off sensors are processed to determine the time delay between the pick-off sensors. The time delay between the two or more pick-off sensors is proportional to the mass flow rate of material flowing through the conduit(s).
Meter electronics connected to the driver generates a drive signal to operate the driver and determines a mass flow rate and other properties of a material from signals received from the pick-off sensors. The driver may comprise one of many well known arrangements; however, a magnet and an opposing drive coil have received great success in the flow meter industry. An alternating current is passed to the drive coil for vibrating the conduit(s) at a desired flow tube amplitude and frequency. It is also known in the art to provide the pick-off sensors as a magnet and coil arrangement very similar to the driver arrangement. However, while the driver receives a current which induces a motion, the pick-off sensors can use the motion provided by the driver to induce a voltage. The magnitude of the time delay measured by the pick-off sensors is very small; often measured in nanoseconds. Therefore, it is necessary to have the transducer output be very accurate.
Generally, a Coriolis flow meter can be initially calibrated and a flow calibration factor along with a zero offset can be generated. In use, the flow calibration factor can be multiplied by the time delay measured by the pick-off sensors minus the zero offset to generate a mass flow rate. In most situations, the Coriolis flow meter is initially calibrated, typically by the manufacturer, and assumed to provide accurate measurements without subsequent calibrations required. In addition, a prior art approach involves a user zero calibrating the flow meter after installation by stopping flow, closing valves, and therefore providing the meter a zero flow rate reference at process conditions.
As mentioned above, in many vibrating sensors, including Coriolis flow meters, a zero offset may be present, which prior art approaches initially correct for. Although this initially determined zero offset can adequately correct the measurements in limited circumstances, the zero offset may change over time due to a change in a variety of operating conditions, mainly temperature, resulting in only partial corrections. However, other operating conditions may also affect the zero offset, including pressure, fluid density, sensor mounting conditions, etc. Furthermore, the zero offset may change at a different rate from one meter to another. This may be of particular interest in situations where more than one meter is connected in series such that each of the meters should read the same if the same fluid flow is being measured.
Therefore, there is a need in the art for a method to determine and compensate for a change in the zero offset of a vibrating sensor. The present invention overcomes this and other problems and an advance in the art is achieved.